Adhesives derived from protein-containing soy flour first came into general use during the 1920's (U.S. Pat. Nos. 1,813,387, 1,724,695 and 1,994,050). Soy flour suitable for use in adhesives was, and still is, obtained by removing some or most of the oil from the soybean, yielding a residual soy meal that was subsequently ground into extremely fine soy flour. Typically, hexane is used to extract the majority of the non-polar oils from the crushed soybeans, although extrusion/extraction methods are also suitable means of oil removal. The resulting soy flour was then denatured (i.e., the secondary, tertiary and/or quaternary structures of the proteins were altered to expose additional polar functional groups capable of bonding) with an alkaline agent and, to some extent, hydrolyzed (i.e., the covalent bonds were broken) to yield adhesives for wood bonding under dry conditions. However, these early soybean adhesives exhibited poor water resistance, and their use was strictly limited to interior applications, thus, they were largely displaced by fossil fuel adhesives that were primarily based on formaldehyde. More recently, there has been a need in the wood adhesives industry to produce more environmentally friendly products, such as those having decreased levels of formaldehyde emissions. Protein adhesives once again become an option, as long as their water resistance could be enhanced. The amine-epichlorohydrin adduct/soy “AE/soy” adhesives fill a need in the marketplace for no added formaldehyde (NAF) adhesives that comply with laws regulating formaldehyde emissions in wood products (California Air Resource Board, “AIRBORNE TOXIC CONTROL MEASURE TO REDUCE FORMALDEHYDE EMISSIONS FROM COMPOSITE WOOD PRODUCTS”, Apr. 26, 2007, and the Formaldehyde Standards for Composite Wood Products Act, Jul. 7, 2010) and to meet voluntary standards for environmentally responsible building such as the U.S. Green Building Council's (USGBC) Leadership in Energy and Environmental Design (LEED) Rating System. AE/soy adhesives also have a marked improvement in water resistance as compared to the traditional soy based adhesives.
Amine-epichlorohydrin polymers (AE polymers) have been used in combination with proteins as adhesives for wood products (U.S. Pat. Nos. 7,060,798 and 7,252,735; U.S. Patent Applications 2008/0021187, 2008/0050602 and 2008/0292886). AE/soy combinations have been demonstrated to be effective and robust adhesives for plywood in commercial systems showing greatly improved performance than traditional soy-based adhesives under both dry and wet conditions. The AE/soy adhesives have been demonstrated to be effective replacements for formaldehyde-based adhesives such as urea-formaldehyde (UF) adhesives. However, there are still some areas where the performance of these materials can be improved. In particular, adhesive strength with certain difficult to bond wood types has been a challenge with these adhesives. Some of the wood types that can be problematical for bonding are maple, hickory and fumed yellow birch. Other areas where the performance of AE/soy adhesives can benefit from additional improvement is in the area of higher bending and stiffness strengths when used as a binder resin for particleboard (PB) or medium density fiberboard (MDF) and also for general adhesive bonds strength improvement under wet conditions. It is also desirable to reduce the level of the AE additive in an AE/soy adhesive since the AE additive is a more expensive ingredient than soy flour.
Wood adhesives based on the combination of soy and AE resins are well known (U.S. Pat. No. 3,494,750; U.S. Pat. No. 7,252,735; U.S. Pat. No. 7,060,798). A number of patent applications have been filed describing improvements to this technology such as the use of low viscosity PAE resins (U.S. Patent publication number 20080050602), the preparation and use of sprayable adhesives for PB and MDF (U.S. Patent publication numbers 2007/073771, 2009/0098387, and 2010/046898), and the use of stain-preventing additives (U.S. Patent publication number 2008/0292886). Although AE/soy combinations have been demonstrated to be effective and robust adhesives for plywood, PB and MDF in commercial systems there are still some areas where the performance of these materials can be improved.